1. Field of the Invention
The present invention relates to power converters that can efficiently operate with a wide input-voltage or output-voltage range.
2. Description of the Related Art
In many applications, power conversion circuits are required to operate with a wide input or output voltage range. For example, most single-phase AC/DC power supplies in today's computer and telecommunication (“telecom”) power systems must operate in the universal AC line range between 90 VRMS and 264 VRMS and must provide a regulated output voltage. Typically, telecom AC/DC power supplies provide a regulated output voltage between 42 V to 58 V, while power supplies for desktop, networking, and server applications provide a single constant output voltage or multiple constant output voltages. However, to further improve energy efficiency, an emerging requirement in server power supplies is a single output voltage selectable from two predetermined levels. Specifically, this new generation of server power supplies with dynamically adjustable output voltage provides a 12V output at full and mid-range loads, and a 6V output at light loads (for light-load efficiency). AC/DC battery chargers are another major class of power converters that operate with a wide input or output voltage range. For example, today's plug-in and battery electric vehicles (EVs) have an output voltage range of 200-450 volts.
There is a trade-off between the input voltage range or the output voltage range and conversion efficiency. Power converters operating in a wide input voltage range or output voltage range exhibit a larger efficiency fall-off than their narrow-range counterparts.
The detrimental effect of a wide input or output voltage range on conversion efficiency is more severe in resonant converters than in pulse-width-modulated (PWM) converters. Generally, resonant converters regulate output voltage by changing switching frequencies. Specifically, in resonant converters designed to provide zero-voltage-switching (ZVS), the switching frequency is increased to maintain output regulation when the input voltage increases or when the output voltage decreases. As a result, these converters suffer from increased switching losses when they operate away from the minimum input voltage or the maximum output voltage. Therefore, resonant converters—including the most efficient series-resonant LLC converter topology—are not suitable for applications with a wide output voltage range. PWM converters that operate at a constant frequency do not suffer from increased switching losses in wide input voltage or wide output voltage applications. Thus, any efficiency drop associated with an output voltage decrease for such converters is primarily due to other factors (e.g., an increased conduction loss resulting from a reduced duty cycle, which increases the RMS current).
The overall efficiency in converters operating in an extremely wide input or output voltage range can be improved by multi-stage conversion. Specifically, a pre-regulator stage can be used to handle wide input voltage variations, and a post-regulator stage can be used to handle the wide output-voltage range. While this approach has been demonstrated to improve efficiency, as compared to a single-stage conversion, it is not preferred as the increased number of components increases circuit complexity and cost.